I think it is unlikely that Triple H will show up at Summerslam in any capacity. It is obvious that he is in no physical condition to wrestle or get involved in any action physically, so bringing him back without any physical interaction would be senseless.
I really hope HHH is not affiliated with the Nexus, because if he is it will be the death of that faction. The whole point is that it is a rogue faction, a unit on own with the total entity being better than any of its individual parts (even though Barrett is clearly the leader and the one with the brightest future). Making Hunter a part of this group puts the focus squarely on him, whether he's a wrestler or a manager/mouthpiece, and by default, takes it off the Nexus guys themselves. So my hope is that he is not a part of Nexus, in fact I predict he'll actually be the mystery GM, who if anything may oppose the Nexus. Especially if his injuries keep him out longer than expected, it would be a way to keep him involved while he cannot compete physically. To be honest, with the duration of his injuries, the fact that it's actually been different injuries, the retirement of HBK, and his family situation, I can't help but wonder if HHH is done, or at least winding down to calling it quits. The mystery GM angle would be a way to do it, keeping him in the business and on camera, but for the most part, out of the ring. Wind his in-ring time down and eventually end it, but keepin him relevant in the process.
Oh, and off topic slightly, the head of Nexus is not Triple H anyway. It will be Michael Cole, with the assistance and the return of Bryan Danielson, to be revealed at Summerslam. And it's going to be vintage.